Catalytic converters are well known for the removal and/or conversion of the harmful components of exhaust gases. While catalytic converters have a variety of constructions for this purpose, one form of construction is a catalytically coated rigid skeletal monolithic substrate, or honeycomb-type element which has a multiplicity of longitudinal channels to provide a catalytically coated body having a high surface area. The rigid, monolithic substrate is fabricated from ceramics and other materials. Such materials and their construction are described, for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830 each of which is incorporated herein by reference.
The monolithic substrate and particularly the multiplicity of channels are coated with a slurry of a catalytic and/or absorbent material (hereinafter referred to collectively as a "coating media"). The slurry contains water and possibly other liquid constituents which upon heating are vaporized. While various methods are known in the art for coating a monolithic substrate with a coating media, such methods from the standpoint of cost are deficient in minimizing the amount of coating media applied, especially when a costly catalytically active precious metal such as platinum, palladium or rhodium is deposited as part of the coating media. Not only is it difficult to coat monolithic substrates, it is also difficult to provide a consistent and reproducible coating pattern within the channels.
One method of coating a prefabricated monolithic substrate is to pump the coating media into the respective channels and then subject the coated substrate to a drying operation. Drying is a necessary step in the coating process so as to remove vaporized constituents (e.g. water vapor) and rapidly fix the coating media in the channels. Systems which employ a drying operation have been unsuccessful in providing a uniform coating thickness and a uniform coating profile wherein the coating media is deposited uniformly over the same length of each of the channels.
It has been proposed to employ a vacuum to draw the coating media upwardly through the channels. For example, Peter D. Young, U.S. Pat. No. 4,384,014 discloses the imposition of a vacuum over the monolithic substrate to remove air from the channels while drawing the coating media upwardly through the channels. The vacuum is then broken and excess coating media is removed, preferably by gravity drainage i.e. excess coating media travels downwardly out of the lower or rearward end of the channels.
James R. Reed et al., U.S. Pat. No. 4,191,126 disclose the dipping of the monolithic substrate in a coating media and utilizing subatmospheric pressure to purge the excess coating media from the surfaces of the support. The applied vacuum is intended to unplug the channels so that the coating media is drawn over the surfaces of each of the channels.
An improvement in these systems is disclosed in Thomas Shimrock et al., U.S. Pat. No. 4,609,563, incorporated herein by reference. This system encompasses a method of vacuum coating ceramic substrate members with coating media wherein predetermined amounts of coating media are metered for application to the ceramic monolithic substrate. The monolithic substrate is lowered into a vessel of preferably predetermined dimensions to a predetermined depth containing the precise amount of coating media which is to be coated onto the substrate. The coating media is then drawn up by a vacuum which is applied to the end of the substrate opposite to the end which is immersed in the coating media bath. No draining or purging of excess coating media from the substrate is necessary nor is any pre-vacuum application step required to eliminate air.
In Shimrock et al., the design of the vessel (also known as a dip pan) containing the precise amount of coating media is desirably shaped to freely receive but closely conform to the substrate to the substrate to be coated. As exemplified is U.S. Pat. No. 4,609,563, if the monolithic substrate is in the shape of an oval, the dip pan is also in the shape of an oval with slightly larger dimensions than the substrate itself.
While the '563 patent process provides a smooth coating exceeding that of the other reference processes, nonetheless, there is still difficulty in obtaining a uniform coating profile wherein the coating media covers the same length of each channel. In addition, because the '563 patent process prefers precisely shaped and dimensioned dip pans for each type of monolithic substrate, added expense is incurred by having to inventory a variety of different sized and shaped dip pans. Still further, the dimensions of each dip pan is preferably only slightly larger than the substrate. Therefore extra care must be taken by operating personnel to place the fragile substrate in the dip pan so that the substrate does not accidently hit the dip pan and be damaged thereby.
An improved vacuum infusion coating method is disclosed in U.S. patent application Ser. No. 08/668,385 filed Jun. 21, 1996 (Attorney Docket No.3983) which seeks to provide a uniform coating profile by applying a vacuum to a substrate partially immersed in a coating media at an intensity and for a time sufficient to draw the coating media uniformly upwardly into the channels of the monolithic substrate. Substrates coated in this manner, while uniformly coated, are difficult to dry. This is because some of the constituents of the coating media vaporize under the coating conditions and a vapor layer forms within the channels. The presence of a vapor layer impedes drying of the coating media and can delay further processing of the coated substrate. Efforts at hastening the drying process by increasing the intensity of the vacuum during vacuum infusion of the coating media can draw the coating media further into the channels than is desired and can result in a non-uniform coating profile.
It would therefore be a significant advance in the art of coating monolithic substrates and particularly monolithic substrates for use in catalytic converters if the drying of the substrate could be hastened while maintaining a uniform coating profile within the channels of the substrate.